Controls for fluorescent lamp lighting systems have been devised and are now commercially available that will turn a light circuit ON and OFF depending on the signals provided by sensors such as motion detectors. Generally, the motion detector is a Passive Infra-Red (PIR) or Doppler technology device that provides a signal whenever motion is detected in the control zone. Motion created by occupants working in the area is not continuous and a detectable signal is not always available for a motion sensor. In an attempt to ensure that the lights are not turned off while the area is still occupied, a manually adjustable time delay or a preset time delay is provided within each sensor to keep the lighting circuit powered for the time delay before turning off the circuit. One disadvantage to circuit-switching occupancy sensors relates to the requirement to manually set a time delay in each sensor. In practice, most such sensors are set to the maximum delay time to minimize the annoyance of having the lights turned off when people are still occupying the area. Having the delay time set to the maximum value reduces the energy saving potential of this control method. A second disadvantage of circuit-switching occupancy sensors relates to having only an ON and an OFF state, the controlled area is either at full light or dark.
Controls for lighting systems have been devised and are now commercially available that will turn a light circuit on and off based on the availability of adequate light from an external source such as daylight. Generally, a photo sensor is provided to look at a window or at a representative area of the workspace to "see" if there is sufficient natural light available. If not, the artificial lighting circuit is switched ON. This type of control is usually provided with a manual adjustment in each sensor to set the activation light level. In order to have a stable control system, the additional light provided by the artificial lights controlled by the circuit must not cause the sensor to immediately turn the lights off again. One disadvantage to circuit-switching photo sensor controls relates to the requirement to manually set the operative light level in each sensor. A second disadvantage of circuit-switching photo sensors relates to having only an ON and an OFF state.
Dimming fluorescent ballasts are commercially available with a light level adjustment control (dimmer) so that the light level can be set manually. The purpose for such controls is primarily for aesthetic purposes rather than for energy saving. Relying on the occupant to set a lower light level to save energy is not a practical method for energy management. The primary purpose for manual dimming of fluorescent fixtures is for aesthetic lighting control and the cost of the ballast and dimmer components does not require economic justification from an energy saving perspective. This type of ballast is too expensive to be implemented in a large scale in a building for the purpose of saving the cost of energy.
Lighting control systems have been devised and are now commercially available that will adjust the light output of a special electronic ballast in response to a control signal provided by a light level controller. Such light level controllers are stand-alone controllers that are connected to a group of light fixtures using extra control wires. These controllers require manual adjustment of the setpoint light level at each sensor. Frequent adjustments of the light sensor is not practical if the light level should be changed daily in order to optimize energy consumption. The significant disadvantages of this type of controller is the high cost of the components because of their individual complexity, the high cost of installation because of their need for distributed control signal wiring to every ballast, the high cost of maintenance due to the requirement for individual calibration and adjustment at each sensor, and that the dimming controller cannot be interfaced to manual controls, timeclocks, or occupancy sensors.
Circuit-Switching controls such as those just described may cause the lighting circuits to be turned ON and OFF many times throughout the day. Most fluorescent lighting systems installed in North America still use magnetic ballasts to power the fluorescent lamps. Recently, fixed output instant start electronic ballasts are being used to replace traditional magnetic ballasts. Magnetic ballasts typically provide a cathode heating circuit in addition to the current to operate the plasma within the lamp. When this type of ballast is turned on, the plasma voltage is applied at the same time as the cathode heating power. It takes about one second before the lamp begins to operate properly. During this starting phase, damage is done to the lamp cathodes that shortens the lamp life. Repeated ON-OFF cycles with a magnetic ballast will dramatically reduce lamp life. Fixed output instant start electronic ballasts generally do not provide auxiliary cathode heat but the cathodes are heated by the plasma current. Again, damage is done to the lamp cathodes until they are indirectly heated to emission temperature. Repeated ON-OFF cycles with instant start ballasts drastically reduces lamp life. For circuit-switching controls applied to conventional magnetic ballast and instant start electronic ballast driven fluorescent lighting circuits, any benefit from the reduced cost of energy is offset by the need for increased lamp replacement.
Present technology dimmable electronic ballasts provide continuous lamp cathode heating which is maintained while the plasma current is reduced to maintain lamp life. There are also electronic ballasts now available that provide a soft start sequence for applying cathode heat before allowing plasma current to flow into the lamps. This soft starting sequence is described in the International Electrotechnical Commission (IEC) standard publication number 929. Many electronic ballasts that claim to have a soft start sequence for the lamps do not conform to the requirements of the IEC standard. For example, a dimming ballast described by Chen et al in U.S. Pat. No. 5,363,020 shows a so-called soft start sequence that does not achieve the intent of the IEC standard because damaging glow current will flow during the heating phase since the plasma voltage is also applied during that time.
Programmable lamp controllers such as described by Luchaco et al in U.S. Pat. No. 5,357,170 can accept input signals from occupancy sensors, light level sensors and manual dimming controls as well as signals from a central time clock, security system and the like. There are three disadvantages to the approach taken by Luchaco, one is that signals from a central time clock and security system require additional signal wiring in the building and the ballasts suggested for use require additional control wiring in addition to the standard power wiring. This additional wiring significantly adds to the cost of installation and increases the payback time. Also, the programmable lamp controllers have setpoints for minimum and maximum light levels and for photo sensor sensitivity that are manually set in the controller at the time of installation. The same disadvantages relating to adapting the control setpoints to optimize energy efficiency apply to this type of controller. The same cost disadvantages of using current technology dimmable electronic ballasts that were primarily designed for aesthetic dimming applications makes their use for the purpose of reducing energy costs not economically viable considering current energy costs.
A further disadvantage of present technology electronic ballasts are that they are fragile with respect to power line voltage transients. It is expensive to apply adequate surge protection in every ballast.
In view of the foregoing discussion, an object of the present invention is to overcome the noted disadvantages of current technology lighting control systems.